Well, there isn’t. There is just Actor model, and Akka actors and Scala actors are two implementations of that model.
All Actor model says that your concurrency primitives are actors, which can:
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receive a message and decide what to do next depending on the content of the message, including:
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send messages to any actors they know about
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create new actors
and provides certain guarantees, e.g.:
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any actor will only handle a single message at a time
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messages sent by actor X to actor Y will arrive in the order thay were sent
There is no difference between Scala and Akka actors on this level.
For differences in what they can do, see Different Scala Actor Implementations Overview. The biggest one, for me, is that Akka supports supervisors and ActorRegistry.